1. Field of the Invention (Technical Field)
The present invention relates to a remote wind sensing instrument for measurements of atmospheric wind. More specifically, the present invention relates to an active acoustic phased array antenna system that measures all three orthogonal components of a three dimensional wind field on a single transmission pulse utilizing simultaneous beams of acoustic waves.
2. Background Art
It is commonly known that changes in the frequency of transmitted electromagnetic or acoustic waves propagating the atmosphere are due to the movements of the atmospheric media. This Doppler effect enables remote measurements of atmospheric wind by transmitting a pulse of signal to illuminate a volume of the atmosphere and then measuring the changes in frequency, called Doppler shifts, of the reflected waves scattered by air in the illuminated volume. In the early development of remote wind sensing instrument, radio waves were first exploited for Doppler shift measurements. Subsequently, after the field of Doppler radar had been well established, acoustic waves were employed for remote wind measurement applications. Many of the principal operations used in acoustic remote wind sensing today had been adopted from the field of Doppler radar.
The term SODAR is an acronym for xe2x80x9cSOund Detection And Rangingxe2x80x9d, and SODAR or Doppler sodar is the term used for a remote wind measurement system utilizing acoustic waves for Doppler shift detection. Measurements that provide the information of atmospheric wind speed and direction as a function of height above the ground are called vertical wind profiles. Doppler radars and sodars are commonly used for vertical wind profiling. Hence, they are often referred to as radar and sodar wind profilers. Typically, a wind profiler is arranged into either a monostatic or bistatic configuration as discussed by Neff, et at., Probing the Atmospheric Boundary Layer, D. H. Lenschow, Editor, American Meteorological Society, Boston, Mass., pp. 201-239, September 1984. A monostatic wind profiling system such as those described in U.S. Pat. No. 4,558,594, to Balser, et al., entitled xe2x80x9cPhased Array Acoustic Antenna,xe2x80x9d U.S. Pat. No. 4,647,933, to Hogg, entitled xe2x80x9cPhased Antenna Array for Wind Profiling Applications,xe2x80x9d and U.S. Pat. No. 5,509,304, to Petermann, et al., entitled xe2x80x9cPhased Array Acoustic Antenna System,xe2x80x9d concerns only with backscatter signals, and uses a common antenna for both the transmission and reception of signals that propagate along the same path. A bistatic wind profiling system such as those described in U.S. Pat. No. 3,889,533, to Balser, entitled xe2x80x9cAcoustic Wind Sensor,xe2x80x9d and U.S. Pat. No. 4,219,887, to MacCready, Jr., entitled xe2x80x9cBistatic Acoustic Wind Monitor System,xe2x80x9d and in J. Appl. Meteo. vol. 15, pp. 50-58, 1976, on the other hand, has different transmission and reception propagation paths, and hence, uses different antennas for the transmission and reception.
The atmosphere absorbs and scatters acoustic waves much more strongly than it does electromagnetic waves. Strong absorption limits the maximum height range of SODAR systems to about 1 km. Strong scattering, on the other hand, provides the advantage of well defined scattering signals contributing to good spatial resolution, a favorable circumstance to employ SODARs for remote wind measurements in a 1-km height range.
Basically, a minimum of three orthogonal components is required for a three dimensional wind vector measurement. Therefore, in a typical monostatic configuration, a wind profiler employs three fixed beams: two tilted beams that are slightly off the vertical to the east-west and north-south directions, and a vertical beam. Two additional tilted beams found on some wind profiling systems are used for consistency check. Doppler shifts in the backscatter signals received on the axis of each beam are interpreted as wind components in the radial direction. The measured components along different axes are transformed into components in the east-west, north-south, and vertical directions resulting in a three dimensional wind vector profile. Prior to the development of a phased array antenna technology, individual antenna was required for each radial wind component measurements. With the advancement in the phased array antenna technology, many wind profilers nowadays employ a single phased array antenna capable of beam steering for the various beams requirement.
The utilization of phased array antenna technology has notably reduced the size and improved the mobility of wind profilers. For example, a single phased array antenna can be employed in place of three separate antennas in a monostatic wind profiling system. Despite this development, however, the technique involved in Doppler wind measurements remains unchanged. For a three-dimensional wind measurement, a monostatic wind profiler obtains radial wind components along a minimum of three fixed beams sequentially on a pulse-by-pulse basis. Following a transmission of a pulse, backscatter signals are received for Doppler wind processing. Backscatter signals from lower height ranges arrive before those from upper height ranges. The time delay for receiving backscatter signal from the highest height range is referred to as a pulse repetition period. A sequence of pulsing is typically arranged in a cyclic order of the number of beams. For example, the pulsing sequence of a wind profiling system employing three fixed beams is: (1, 2, 3), (1, 2, 3), . . . (1, 2, 3). A pulse repetition period of one beam must be completed prior to an initiation of the next pulse repetition period in the sequence. If the pulse repetition periods are overlapped, signals from the current pulse will start to be received while signals from the previous pulse are still arriving. Unless some other information is available, it is not possible to interpret these signals.
For wind profilers employing radio waves that propagate the atmosphere at a speed of light, approximately 3xc3x97108 m/s, a pulse repetition period is of an order of 10 xcexcs and is considered insignificant. However, for wind profilers employing acoustic waves that propagate the atmosphere at a relatively slow sonic speed, approximately 340 m/s, the pulse repetition period is large and becomes a significant performance load factor. For example, it takes 2 full seconds to retrieve backscatter signals from a height range of 340 m (i.e., a round trip distance of 680 m). To complicate the matter, because the individual radial wind components are temporally separated from one another by at least one pulse repetition period, averaging of these wind components over many consecutive pulsing sequences is required in order to obtain meaningful wind measurements. Thus, the propagation delay associated with the retrieval of backscatter signals in the sequential pulsing operation becomes a significant problem for SODARs to achieve a high temporal resolution.
In a conventional spectral processing, spectral estimation is implemented by a discrete frequency analysis using a Fast Fourier Transform (FFT). The detectability of a signal peak is enhanced by an incoherent spectral averaging process that averages a number of consecutive power spectra to smooth out the noise floor and better define the signal peak for a greater measurement resolution. Because the incoherent spectral averaging process does not increase the signal-to-noise ratio (S/N), a large number of spectra is required for average processing. Typically, for spectral processing of SODAR signals, a minimum of 20 spectra is required for each radial wind component. This translates to a minimum of 60 pulse repetition periods for measurements of a three dimensional wind profile. Thus, in the conventional spectral processing, a greater spatial resolution is rendered at an expense of a lower time resolution resulting from the many pulse repetition periods required in the spectral averaging process.
It is seen from the discussions presented that the signal retrieval technique originally developed for Doppler radars is inefficient for used with Doppler sodars due the vast difference in the propagation speed between electromagnetic and acoustic waves. Furthermore, long averaging periods are required to contend with the incoherent noise retained in the conventional spectral processing resulting in a poor measurement time resolution. The two components that can significantly improve the performance of wind profilers employing acoustic waves for Doppler measurements are therefore: (1) a more efficient signal retrieval technique that eliminates the propagation delay associated with the sequential pulsing operation to significantly increase the measurement time resolution, and (2) an improved spectral processing technique that eliminates the need for a time consuming incoherent spectral averaging process to significantly increase the spatial resolution of the measurements without decreasing the time resolution.
The present invention is an active acoustic phased array antenna system that simultaneously measures all three orthogonal components of a three dimensional wind vector profile on a single transmission pulse to significantly increase the measurement time resolution. The system accomplishes this task by transmitting a broad beam acoustic pulse to illuminate a zone of the atmosphere, and then measuring Doppler shifts in the backscatter signals arrived at the system from various directions along the broad beam projection utilizing simultaneous receiving beams of narrow beamwidth.
A HYPER-SODAR employs separate transmitting and receiving array antennas that are co-located in the same antenna enclosure. Because of the separate transmitting and receiving antennas employed, HYPER-SODAR is classified as a bistatic system. The transmitting array antenna is used to transmit a broad beam acoustic pulse to illuminate a zone of the atmosphere producing scattered waves along the propagation path. The receiving array antenna is used to produce multiple beams of narrow beamwidth for simultaneous reception of backscatter signals from multiple directions along the broad beam projection, eliminating the propagation delay associated with the sequential pulsing operation. The simultaneous beams are obtained by means of a hybrid of analog and digital beamforming technologies that enables simultaneous forming of a programmable number of receiving beams in any look-directions within the designed fields-of-view.
A preferred embodiment of the present invention comprises a hyper-sound detection and ranging system for high-resolution remote wind measurements comprising a transmitting array of acoustic transducer elements for transmitting a pulse of broad beam acoustic waves toward a zone spaced from the transmitting array, signal transmitting means comprising a signal generating means for generating a transmit signal, and an amplifier for amplifying the transmit signal for driving the elements in the transmitting array, a receiving array of acoustic transducer elements for simultaneous reception of reflected acoustic waves scattered by air in the zone from a plurality of directions along the broad beam projection utilizing a plurality of simultaneous receiving beams of narrow beamwidth, and signal receiving means comprising a hybrid of analog phased array processing means and digital phase array processing means for simultaneous forming of the receiving beams and providing of the plurality of simultaneous receiving beams in multiple planes on the receiving array for simultaneous reception of the reflected acoustic waves. Preferably, the transmitting array and receiving array are co-located in a common array antenna enclosure, and preferably the transmitting array elements are arranged in a taper configuration providing K rows and L columns of said elements, where K and L are integers, and said K can be equal said L. Preferably, the receiving array elements are arranged in a taper configuration providing M rows and N columns of said elements, where M and N are integers, and said M can be equal said N, and more preferably are electrically grouped row-wise into M rows providing M independent channels referencing to a common ground. The analog phase array processing means preferably comprises M independent phased array processing circuits referencing to the common ground and corresponding to the M independent channels on a one-to-one basis. Preferably, each of the phased array processing circuits comprises a plurality of phase shifting means for phase shifting the elements in corresponding channel, the phase shifting means capable of providing a plurality of element-to-element progressive phase shifts on the channel. The plurality of element-to-element progressive phase shifts on each channel preferably comprise: xcfx861, xcfx862, xcfx863, . . . , xcfx86P where P is an integer, providing P sets of concurrent element-to-element phase shifts of {(xcfx861, 2xcfx861, 3xcfx861, . . . ), (xcfx862, 2xcfx862, 3xcfx862, . . . ), (xcfx863, 2xcfx863, 3xcfx863, . . . ), . . . (xcfx86P, 2xcfx86P, 3xcfx86P, . . . )} on the channel. In a preferred embodiment, P equals 2, xcfx861 equals 0xc2x0, and xcfx862 comprises at least one value selected from the group consisting of xe2x88x9290xc2x0 and 90xc2x0, and the values provide concurrent element-to-element phase shifts of (xcfx861, 2xcfx861, 3xcfx861, . . . ) and (xcfx862, 2xcfx862, 3xcfx862, . . . ) on each channel. Preferably, each phased array processing circuit further comprises a plurality of signal summing means for summing the plurality of phase shifting means outputs, the signal summing means capable of providing a plurality of phase-shifted sum signals, a plurality of bandpass filters for filtering the plurality of phase-shifted sum signals, the bandpass filters capable of providing a plurality of filtered phase-shifted sum signals, a multiplexer for multiplexing the filtered phase-shifted sum signals, and a sample and hold circuit for synchronized sampling the multiplexer output, the circuit capable of providing filtered phase-shifted sum signal that is synchronized with the filtered phase-shifted sum signals from all other channels.
In a preferred embodiment of the present invention, the digital phase array processing means comprises an M-channel analog-to-digital converter for digitizing outputs of sample and hold circuits from M channels, and a digital phase synthesizer for processing the digitized outputs, wherein the converter and the synthesizer are capable of providing simultaneous receiving beams of narrow beamwidth. Preferably, the digital phase synthesizer comprises a digital beamforming algorithm comprising zero padding of the digitized outputs from M channels, wherein the zero padding provides zero padded data having a vernier sampling interval required for a desired delay quantization, delayed sum of the zero padded data from M channels with NB imposed delays on each channel, where NB is an integer whose value is at least three, providing NB sets of delayed sum data, and interpolation of NB sets of delayed sum data by means of utilizing a finite impulse response (FIR) digital filter obtaining NB sets of signals representing reflected acoustic waves along the projections of NB simultaneous receiving beams. Preferably, the system further comprises a digital computer for controlling the M-channel analog-to-digital converter and for executing the digital beamforming algorithm.
A preferred embodiment of the present invention also comprises a co-spectrum method comprising a co-spectral processing algorithm for computing the co-spectra of signals representing reflected acoustic waves received along each receiving beam of a wind profiling system. Preferably, the co-spectral processing algorithm comprises the steps of utilizing the signals received from the current and immediate previous pulses for the co-spectral processing, and computing the mean frequencies of the co-spectra.
A preferred embodiment of the present invention further comprises a method for determining a three dimensional wind vector profile from a plurality of radial wind components measured on a single transmission pulse, utilizing a hyper-sound detection and ranging system and a co-spectrum method, the method comprising steps of (a) transmitting a pulse of broad beam acoustic waves toward a zone spaced from the transmitting array; (b) producing NB simultaneous receiving beams of narrow beamwidth, where NB is an integer whose value is at least equal to three, for simultaneous reception of signals representing reflected acoustic waves scattered by air in the zone along the projections of the NB receiving beams utilizing the receiving array and signal receiving means; (c) storing the signals in step (b) into buffer #1; (d) repeating steps (a) and (b), and storing the signals in step (b) into buffer #2; (e) computing the NB sets of mean frequencies along the axes of the NB simultaneous receiving beams by means of a co-spectrum method utilizing the signals in buffers #1 and #2; (f) interpreting the differences between each set of the mean frequencies along the receiving beam and the transmitting frequency as wind components in the radial direction; (g) transforming the wind components along the NB different axes into wind components in the east-west, north-south, and vertical directions resulting in a three dimension wind vector profile; (h)repeating steps (a), (b), (c), (e), (f), and (g); (i) repeating steps (d), (e), (f), and (g); and alternately repeating between steps (h) and (i) for a continuous operation. The method preferably comprises utilizing a remote wind sensing instrument comprising a transmitting array of acoustic transducer elements for transmitting a pulse of broad beam acoustic waves toward a zone spaced from said transmitting array; signal transmitting means comprising signal generating means for generating a transmit signal, and an amplifier for amplifying said transmit signal for driving said elements in said transmitting array; a receiving array of acoustic transducer elements for simultaneous reception of reflected acoustic waves scattered by air in the zone from a plurality of directions along the broad beam projection utilizing a plurality of simultaneous receiving beams of narrow beamwidth; and signal receiving means comprising a hybrid of analog phased array processing means and digital phase array processing means for simultaneous forming of said receiving beams and providing of said plurality of simultaneous receiving beams in multiple planes on said receiving array for simultaneous reception of the reflected acoustic waves. Preferably, a spectral processing method is utilized, comprising the steps of receiving reflected acoustic waves along a plurality of simultaneous receiving beams of a wind profiling system; utilizing the signals received from the current and immediate previous pulses for the co-spectral processing; and computing the mean frequencies of the co-spectra.
A primary object of the present invention is to provide a new and more efficient backscatter signal retrieval technique that eliminates the propagation delay associated with the sequential pulsing operation of wind profilers employing acoustic waves to significantly increase the measurement time resolution.
Another object of the present invention is to provide an improved spectral processing technique for Doppler frequency extractions that dismisses the need for a time consumed incoherent spectral averaging process to significantly increase the spatial resolution of the measurements without decreasing the time resolution.
A primary advantage of the present invention is the improved efficiency of signal retrieval;
Another advantage of the present invention is the elimination of propagation delay in signal retrieval; and
Yet another advantage of the present invention is the elimination of the spectral averaging process.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.